1. Field of the Invention
The present invention relates to a preamplifier used in a high speed optical fiber communication system, and more particularly to a trans-impedance type preamplifier in which an electrical current signal is amplified through electrical current-voltage conversion. Still more particularly, the present invention relates to the technology by which various characteristics are improved efficiently, in which a wide band, a wide dynamic range, and high impedance are made possible.
2. Description of Related Art
In a receiver of an optical fiber communication system, a light signal carried through an optical fiber is converted to electrical current by a photo diode as a photoelectric conversion device. This electrical current signal is output as a voltage 6value by a preamplifier's current-voltage conversion. In this preamplifier, characteristics such as low noise, a wide dynamic range, a wide band, and high gain are required.
In order to satisfy such requirements, an amplifier called a trans-impedance type preamplifier is generally used. A typical trans-impedance type preamplifier includes a feedback resistance connected to input and output terminals.
FIG. 1 is a circuit diagram showing the preamplifier for a conventional optical communication system, and a photo diode (PD is used hereinafter) as a photoelectric conversion device in an optical communication receiver. In this circuit, a PD outputs electric current corresponding to the intensity of a received light signal, the preamplifier converts this electric current signal to a voltage signal through impedance conversion, and the signal is then amplified. The following equation is well-known for describing the relationship between input electric current Iin input to the trans-impedance type preamplifier and output voltage Vout output from the trans-impedance type preamplifier when Rf is defined as a resistance value of the feedback resistance: EQU Vout=-Iin.multidot.Rf.
In accordance with this equation, sufficient sensitivity, namely sufficient output voltage amplitude, can be obtained, by using the larger value of the feedback resistance Rf.
However, if a resistance value Rf is large, the output amplitude is saturated and the output waveform is then distorted, in the case where the amplitude of the electrical current is large. In the trans-impedance type preamplifier, it is one solution method that a small value of an equivalent resistance of the feedback resistance is used, when electrical current of the input signal is large, in order to improve the dynamic range characteristics.
As shown in FIG. 1, a source and a drain of a feedback transistor are connected in parallel to the feedback resistance, and the combined equivalent resistance value of the feedback resistance and the feedback transistor is defined as total feedback resistance of the preamplifier. Further, a control circuit of the drawing consists of a source follower circuit. When input electrical current is small, the feedback transistor is completely closed, a combined feedback resistance value is equal to the complete feedback resistance.
On the other hand, when input electrical current is large, since the control circuit changes gate voltage of the feedback transistor in accordance with the value of input current, the value of the above-described combined feedback resistance, namely, the current-voltage conversion gain can be changed and thus a wide dynamic range can be obtained.
In order to obtain a preamplifier having a high conversion gain of more than 1 K.OMEGA. (approximately 60 db.OMEGA.) trans-impedance and a band width of ten or slightly more GHz, it is necessary that the gain-bandwidth product makes the range bigger. At the same time, it is important to utilize a peaking characteristic based on the feedback circuit.
For a frequency lower than several GHz, a phase of an output terminal of an amplifier shown in PIG. 3 is inverted to a phase of an input terminal thereof, and the type of feedback by the feedback resistance is a negative feedback. However, when the frequency is 10 GHz or more, delay of a feedback loop is unavoidable and thus the feedback by the feedback resistance becomes nearly a positive feedback due to the phase delays. That is to say, peaking phenomenon occurs.
Peaking phenomenon is used as a design method to expand band width in a high frequency region, by utilizing a phenomenon in which the closer the negative feedback is to the positive feedback, the more the conversion gain increases.
Here, band-width fw of the amplifier circuit is in general expressed by the following equation: EQU fw=1/(2.pi.Rin.multidot.Cin)=A/(2.pi.Rf.multidot.Cin),
where Rin is an input resistance, Cin is an input capacitor (PD's junction capacity, amplifier's input capacity, and floating capacity loaded in practice), and A is a close loop gain of the amplifier circuit.
In a conventional preamplifier as shown in FIG. 1, when input electrical current is large, the equivalent resistance of the feedback resistance Rf is small, and thus the band width grows in accordance with the above-described equation. However, there has been a problem where, in a high conversion gain and wide band circuit using the above-described peaking effect, when large electrical current is input, it has been impossible to make a wide band of a frequency having a wide dynamic range, since output voltage is distorted or oscillated because of the obvious peaking phenomenon.